As a method for producing hollow polymer particles having single holes, there is a method involving preparing a monomer solution by evenly dissolving a hydrophilic monomer, a crosslinkable monomer, and an oil-soluble solvent together with a polymerization initiator, emulsifying and dispersing the monomer solution in an aqueous phase, and thereafter carrying out polymerization. This production method is a method for forming single holes in the particles based on the phase separation effect of the polymer to be produced and the oil-soluble solvent.
However, the hollow polymer particles having single holes which are obtained by this method have particle diameter distribution dependent on the mechanical operation method of the emulsification and dispersion. Therefore, there is a problem that control of the outer diameter (particle diameter), the inner diameter (diameter of the single hole), and the ratio of the outer diameter and the inner diameter within a certain range is difficult.
Even if classification operation is performed with sieves or meshes to make the outer diameters even, it is difficult to obtain hollow polymer particles with sufficiently even outer diameter distribution. Further, even if the outer diameters are made even, it is impossible to make the inner diameters even.
Furthermore, also known is a method for classification of obtained hollow polymer particles in a fluid dynamic manner based on a difference in specific gravity or the like. However, particles with a large outer diameter and a large inner diameter (high degree of hollowness) and particles with a small outer diameter and a small inner diameter (low degree of hollowness) have similar mobility, so that these particles cannot be classified by this method.
To deal with such a problem, a method for producing hollow polymer particles by making seed particles absorb monomer components and then carrying out polymerization has been investigated. It is supposed to be possible to produce hollow polymer particles with relatively even outer diameters by this method.
For example, Patent Document 1 discloses a method for producing polymer particles having single inner holes. The method includes the steps of dispersing polymerizable monomer components including a crosslinkable monomer, a hydrophilic monomer, and other monomers in an aqueous dispersion medium in the presence of different polymer particles with a different composition from that of a copolymer of these polymerizable monomer components to make the different polymer particles absorb the polymerizable monomer components and then polymerizing the polymerizable monomer components. In Patent Document 1, polystyrene, and copolymers of styrene with at least one kind selected from acrylic acid, methacrylic acid, acrylic esters, methacrylic esters, and butadiene are exemplified as the above-mentioned different polymer. Further, divinylbenzene, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, and the like are exemplified as the above-mentioned crosslinkable monomer. Acrylic acid, methacrylic acid, methyl methacrylate, 2-hydroxyethyl methacrylate, vinylpyridine, glycidyl acrylate, glycidyl methacrylate, and the like are exemplified as the above-mentioned hydrophilic monomer. Styrene and the like are exemplified as other monomers. However, it is difficult to obtain hollow polymer particles with sufficiently even outer diameters and inner diameters by the method disclosed in Parent Document 1. In the examples of Patent Document 1, although hollow polymer particles with single holes are generally obtained, both of the outer diameter and the inner diameter are not even, and not only particles with a single hole structure but also particles having a plurality of holes are present in the particles.
Patent Document 2 discloses a method for producing hollow polymer particles having a single inner hole with an average inner hole diameter 0.25 to 0.8 times that of the particles. The method includes emulsion polymerization of monomer components (B) including an ionic monomer (B-a), a nonionic monomer (B-b) with a solubility parameter decreasing along with the change from the monomer to a polymer at the time of polymerization, and a nonionic monomer (B-c) other than the nonionic monomer (B-b) in an aqueous medium at a polymerization temperature satisfying a certain condition with the use of a water-soluble polymerization initiator. The emulsion polymerization is carried out in the presence of polymer particles (A) obtained by polymerization of monomers including an ionic monomer (A-a), a nonionic monomer (A-b) with a solubility parameter which does not change or increases along with the change from the monomer to a polymer at the time of polymerization, and a nonionic monomer (A-c) other than the nonionic monomer (A-b). Further, in the production method disclosed in Patent Document 2, among the combinations of the polymer particles (A) and the monomer components (B), there is a characteristic that the absolute value of the difference between the solubility parameter [δ(A-b),p] of the polymer obtained by polymerization of the nonionic monomer (A-b) component and the solubility parameter [δ(B-b),m] of the nonionic monomer (B-b) component is 1.0 or lower. However, it is impossible to obtain hollow polymer particles with sufficiently even outer and inner diameters by the method disclosed in Patent Document 2. Moreover, since a water-soluble polymerization initiator is used, the production method disclosed in Patent Document 2 has a problem that not only the polymerization ratio is lowered since the content of the polymerization initiator in the monomer oil droplets is low but also emulsion polymerization is caused simultaneously in the aqueous phase so that particles with no inner hole are mixed together.
Patent Document 3 discloses a method for producing particles having an oily substance as inner nuclei by injecting an evenly mixed solution A containing a hydrophilic monomer, a crosslinkable monomer, another monomer, and the oily substance into a liquid B which is immiscible with the solution A through a micro-porous membrane to obtain oil droplets and thereafter carrying out polymerization. However, the method of passing the mixed solution through a micro-porous membrane has a problem that ultimately, operation such as classification has to be carried out, although the method makes the distribution of the outer diameter even as compared with other conventional methods using emulsification devices.